Field of the Invention and Related Art Statement
The present invention relates to method and apparatus for measuring a phase difference of a reflection type opto-magnetic record medium including a substrate and a magnetic record layer applied on the substrate, and more particularly to a method and apparatus for measuring a phase difference due to the birefringence of the substrate and a phase difference due to the Kerr elliptic conversion of the magnetic record layer of the opto-magnetic disk.
In an apparatus for reproducing an information signal recorded in a reflection type an opto-magnetic record medium such as opto-magnetic record disk, linearly polarized light is made incident upon the opto-magnetic disk and the information signal is read out or reproduced by detecting the rotation of the polarization plane of the linearly polarized light reflected from the opto-magnetic disk due to the Kerr effect. An amplitude of the reproduced signal is proportional to sin 2.theta..sub.k ', where .theta..sub.k ' is the Kerr rotation angle. Usually the opto-magnetic record disk comprises a substrate made of dielectric material and a record layer made of magnetic material applied on the substrate. In the reproducing apparatus using the opto-magnetic disk, the linearly polarized light is made incident upon the magnetic record layer of the disk through a face of the substrate, so that the light reflected by the magnetic record layer of the opto-magnetic disk is transmitted through the substrate twice. Therefore, the linearly polarized light is converted into elliptically polarized light due to the birefringence of the dielectric substrate which is generally made of synthetic resin such as PMMA (polymethyl methacrylate) or PC (polycarbonate). Further the magnetic layer of the opto-magnetic record disk has such a property that the linearly polarized light is converted into elliptically polarized light. Hereinafter, this is called the Kerr elliptic conversion. That is to say, when the linearly polarized light is made incident upon the magnetic record layer and is reflected thereby, the linearly polarized light is converted into elliptically polarized light and this Kerr elliptic conversion also introduces the phase difference. Due to these phase differences of the opto-magnetic disk, the effective Kerr rotation angle is reduced and thus the amplitude and C/N of the reproduced signal are decreased to a large extent. In the present specification, a sum of the above explained phase differences is sometimes called the total phase difference of the opto-magnetic record medium.
In order to obtain the reproduced signal having a sufficiently large amplitude and C/N, it would be necessary to measure the total phase difference of the opto-magnetic record medium and to manage the manufacturing of the disk. There have been proposed various methods of measuring the phase difference of the opto-magnetic record medium. FIG. 1 is a schematic perspective view showing a known apparatus for measuring the phase difference of the opto-magnetic disk. An opto-magnetic disk 1 is held stationary and a parallel light beam of linearly polarized light is made incident upon the substrate 1 via a polarizer 2 and light reflected by the disk 1 is made incident upon a light receiving element 4 by means of an analyzer 3 which is arranged rotatably. By rotating the analyzer 3 it is possible to measure the phase difference of the opto-magnetic disk 1, i.e. the phase difference between P-polarized light and S-polarized light. However, in this known apparatus, since the parallel light beam is made incident upon the opto-magnetic disk 1, it is impossible to measure the phase difference of the disc accurately, because the birefringence of the substrate varies in accordance with the incident angle of the light. Further, in the known apparatus the measurement is carried out while the opto-magnetic disk 1 is held stationary, so that the actual influence of the phase difference of the disk upon the reproduced signal can not be determined and moreover the measurement can not be effected for the whole surface of the disk 1.
There has been further proposed a known method in which linearly polarized light is made incident upon a substrate and light transmitted through or reflected by the substrate is detected via a phase compensator. However, the known method can not measure the total phase difference of the opto-magnetic disk. That is to say, in the known method, linearly polarized light is made incident upon the substrate and the phase difference due to the birefringence of the substrate is measured. In practice, knowing only the phase difference due to the birefringence of the substrate can not ensure accurate manufacture of the disks. Particularly, in the known phase difference measuring apparatus it is impossible to detect the influence of the phase difference upon the information signal read out of the disks. Therefore, even if the disk is manufactured such that the phase difference of the disk due to the birefringence of the substrate becomes small or zero, it is impossible to obtain the information signal having large signal level and C/N. Further, also in this known phase difference measuring method, the measurement is carried out, while the substrate is held stationary, so that the measurement can not be performed over the whole surface of the substrate. In this manner in the known phase difference measuring method, only the phase difference of the substrate due to the birefringence thereof can be measured locally, so that the disk can not be evaluated correctly.